Mothers teach newborn babies what to be scared of by literally letting them smell their fear, new research suggests.
In a study of rats, scientists showed how a mother's alarm scent primed her days-old offspring to develop a peppermint phobia.
They believe the same "smell of fear" mechanism may be at work in humans.
For generations experts have been puzzled by the degree to which a mother's traumatic experiences can affect her children.
Some children of Holocaust victims, for instance, react as if they had witnessed the same horrors as their mothers.
Their fear, manifested in the form of nightmares, avoidance behaviour and even flashbacks, appears too deeply ingrained to have been learned simply by hearing stories.
Dr Jacek Debiec, from the University of Michigan, said: "Our research demonstrates that infants can learn from maternal expression of fear.
"Before they can even make their own experiences, they basically acquire their mothers' experiences. Most importantly, these maternally-transmitted memories are long-lived, whereas other types of infant learning, if not repeated, rapidly perish."
Polish-born Dr Debiec found evidence of human babies "inheriting" fear from their parents while working with the grown children of Jews who survived the Nazi death camps.